PARIS -- I sensed that something was different as my wife and I emerged into the soft night air from a restaurant on the Quai d'Orsay, the Left Bank roadway paralleling the Seine River.

It was about 10:30 on a Friday night in May and the riverside road (think Memorial Drive or Storrow Drive) should have been bustling with traffic. It was empty, but that didn't register immediately. Then, on the empty expanse of boulevard, two in-line skaters whizzed past. Puzzled, and finally aware of the abnormal stillness, we looked farther up the road and saw the approaching blue lights of police motorcycles.

Then our puzzlement turned to astonishment. Behind the police came a swarm of in-line skaters -- hundreds, mostly young, more men than women, their speeding skates skimming over the road to create a low, almost eerie hum.

There seemed to be little shouting or talking as the skaters bent to their task, reaching speeds, we later learned, of about 15 to 20 miles per hour and up to 30 on some downhill stretches. One carried a large Swiss flag; some had colored lights on their wheels, a nice effect. Most had some body padding, but fewer had helmets. One of our first thoughts was the potential for injury. As in any herd, if one of them fell, surely others would follow in a dangerous pileup. None did as we watched, but it does happen, we found out.

It seemed the parade of skaters might never end, but then some trailing blue lights appeared, along with an ambulance or two. We didn't know it at the time, but we had witnessed the weekly Pari Roller, a phenomenon that is billed as the world's largest weekly street-skating event.

It takes place every Friday night, except when it rains, and can attract as many as 20,000 skaters. Group skates have been around awhile in the United States and Europe, especially since the advent of in-line skates, which provide speed and maneuverability not possible with quad roller skates. But, in scale and longevity, nothing matches the festive Parisian skates, which began in a small way in 1994 and quickly grew to a point where, in 1997, the police decided to become involved for the safety of everyone concerned -- skaters, motorists, and onlookers. Today, the police not only block off roads and provide an escort for the skaters, but about 20 officers on skates join the rolling ranks during the weekly "rando," derived from the French word "randonée," or tour.

Paris, a city of boulevards, is particularly suited to what some call the "Friday night fever" because the roadways are wide enough to allow phalanxes of skaters to maintain their speed and generally smooth enough to ensure a fast ride.

Before you pack your skates, however, heed a warning. The Friday night skate is not for everyone. Skaters are expected to be able to go fast enough to keep up with the pack, maneuver, and come to a quick stop. Pari-Roller, the group that organizes the skates, makes these points on its website (pari-roller.com) : "WARNING: From its origins, [this event] is meant for [experienced] skaters. Route includes technical difficulties on purpose . . . and the speed of the procession is rather fast. . . . It is essential to your safety and that of the other participants that you KNOW HOW TO BRAKE."

It adds: "IF YOU FALL, don't leave your hands on the ground, and try to get up as quickly as possible. . . . Wear protections, especially wrist pads (80 percent of all breaks occur in the wrists ) ."

Well, you get the idea.

The group lists "Rules of the Game," emphasizing that "humility and anticipation" are required to navigate the course safely.

One Internet blogger from the United States who regarded the skate as an interesting way to see Paris by night was quickly disillusioned: "Everything was fine until the skate started and I was at the end of the pack of hundreds of skaters. Skate employees [marshals] bring up the rear and 'herd' the group down the streets. Because I was at the end (still going very fast I might add) employees started to yell at me frantically in French. . . . I was kicked out of the skate for being in the end of the pack and not going what they consided to be fast enough. . . . I planned for months in advance to take this skate. I don't want to totally bash it, so I will say, skate if you must, but be sure you are an EXCELLENT and SWIFT skater ."

By most accounts, skate nights produce only a handful of injuries -- occasionally some broken bones -- but mostly bruises, scrapes, or wrist injuries from breaking falls.

And because equality is part of the French national motto, a more modest street skate, for inexperienced skaters and families, is held Sunday afternoons starting at the Place de la Bastille.

The Friday night skate is also a spectator sport, particularly in the center of the city, where young women are said on occasion to upend buckets of water from their apartments above the route to cool down skaters on hot nights.

We never did find out why the two skaters we saw first were so far ahead of the pack but surmised they were outriders checking for road debris or other obstacles. The skaters have other methods of alerting those in back to sudden stops or other problems, such as raising their arms. The rules insist that riders stay off sidewalks, but two of them skated through a break in the curb to whip past us in a bit of derring-do.

But that was an exception, because the Friday skate is no Roller Derby. Otherwise there would be chaos instead of the good-natured good time that just about everyone seemed to be having. Supposedly, even Paris motorists show forbearance for the traffic delays. Try that in Boston.

If You Go

The skate begins every Friday night (weather permitting) at 10 at Place Raoul Dautry in the 14th arrondissement, between the Montparnasse office tower and the Paris-Montparnasse train station. Supervised by yellow-shirted Pari-Roller marshals, the skate lasts about three hours, including a break for wine or other refreshment, returning to Montparnesse at 1 a.m.

The route varies from week to week, but covers about 18 1/2 miles, mostly through central Paris, including roads along the Seine. Although the tour is free and open to anyone who "can control his or her speed," according to Pari-Roller, the group that organizes the skates (pari-roller.com ), it does seek memberships through contributions that help to cover expenses and also provide accident insurance. The group's address is Pari-Roller, Adhesions, 16 Bd Saint Germain, 75005 Paris, France.